How to measure face size in meters, centimeters or any units which is set?
Simply tick Length  under Mesh Display in the Properties side panel and it will show the length of the edges (also tick Area to show the area if there are any faces). These measurements will use the unit specified under Units in your Scene settings. The shown units are local units and are scaled to world units by the object's Scaling settings (see also Why do the measurements of this object seem erroneous?).



For something more extravagant, you can try the built-in Ruler/Protactor tool or Dolf's Caliper addon (install and usage instructions on the page).